Micromechanical devices are known having a mirror structure obtained with the technology of semiconductor materials.
These micromechanical devices are used in portable apparatus, such as for example portable computers, laptops, notebooks (including ultra-thin notebooks), PDAs, tablets, cellphones, and smartphones, for optical applications, in particular for directing light beams generated by a light source, as desired.
By virtue of their reduced dimensions, these devices are able to meet stringent requirements as regards bulk, as to area and thickness.
For example, micromechanical mirror devices are used in miniaturized projector modules (the so-called pico-projectors), which are able to project images at a distance or to generate desired patterns of light.
In combination with an image-capture module, a projector module of this kind makes it possible, for example, to produce a three-dimensional (3D) photo or video camera for obtaining three-dimensional images.
Micromechanical mirror devices generally include a mirror element suspended over a cavity and manufactured from a body of semiconductor material so to be mobile, for example with a tilting or rotation movement, for directing the incident light beam in any desired way.
For example, FIG. 1 schematically shows a pico-projector 9 comprising a light source 1, typically a laser source, generating a light beam 2 made of three monochromatic beams, one for each base color, which, through an optical system 3 represented only schematically, is deflected by a mirror element 5 in the direction of a screen 6. In the example shown, the mirror element 5 is of a two-dimensional type, driven so as to rotate about a vertical axis A and a horizontal axis B. Rotation of the mirror element 5 about the vertical axis A generates a fast horizontal scan, as shown in FIG. 2. Rotation of the mirror element 5 about the horizontal axis B, perpendicular to the vertical axis A, generates a slow vertical scan, typically of a sawtooth type.
The scan scheme obtained is shown in FIG. 2 and designated by 7.
Rotation of the mirror element 5 is driven via an actuation system, which is currently of an electrostatic, magnetic, or piezoelectric type.
For example, FIG. 3 shows a mirror element 5 with a purely electrostatic actuation. Here, a chip 10 comprises a platform 11 suspended over a substrate (not visible), having a reflecting surface (not shown), and supported by a suspended frame 13 through a first pair of arms 12 (first torsion springs). The first arms 12 extend on opposite sides of the platform 11 and define the rotation axis A of the mirror element 5. The suspended frame 13 is connected to a fixed peripheral portion 15 of the chip 10 via a second pair of arms 16 (second torsion springs), which enable rotation of the suspended frame 13 and the platform 11 about the horizontal axis B. The first and second arms 12, 16 are coupled to respective actuation assemblies 18A, 18B of an electrostatic type. Each actuation assembly 18A, 18B here comprises first electrodes 19 facing respective second electrodes 20.
In detail, the first electrodes 19 are fixed with respect to the respective arms 12, 16 and are combfingered with the second electrodes 20 for generating a capacitive coupling. By virtue of the arrangement of the electrodes 19, 20 of each actuation assembly 18A, 18B, the drive structure is also defined as “comb drive structure”.
By applying appropriate voltages between the first electrodes 19 and the second electrodes 20, it is possible to generate attraction/repulsion forces between them and thus cause rotation of the first electrodes 19 with respect to the second electrodes 20 and torsion of the arms 12, 16 about the respective axes A, B. In this way, controlled rotation of the platform 11 about axes A, B is obtained, and thus scanning in the horizontal direction and in the vertical direction.
Rotation of the mirror element 5 about the vertical axis A that produces horizontal scanning is obtained with an angle generally of ±12° in a resonating way at 25 kHz, and rotation of the mirror element 5 about the horizontal axis B that produces vertical scanning is generally obtained with an angle of ±8° at 60 Hz, suitable for the frame rate of video signals. As mentioned, due to the lower vertical scanning rate, it may be driven in a quasi-static, non resonant, way.
It has already been proposed to control the scanning movement about at least the horizontal axis B in a piezoelectric way. For example, in the device described in United States Patent Application Publication No. 2011/0292479, the suspended frame is connected to the fixed structure via spring elements having a coil shape formed by a plurality of arms that are mutually parallel and arranged along each other. Each arm carries a piezoelectric band, and adjacent piezoelectric bands are biased at opposite polarity voltages (for example, ±20 V). Due to the characteristics of piezoelectric materials, this biasing causes deformation in opposite directions (upwards and downwards) of adjacent arms and rotation of the frame and the platform in a first direction about the horizontal axis B. By applying an opposite biasing, rotation of the frame and of the platform in a second direction, opposite to the first, is obtained. Vertical scanning may thus be obtained by applying alternating bipolar voltages to the arms. A similar actuation system here controls also horizontal scanning.
Application of alternating bipolar voltages reduces, however, the service life of the mirror element 5.
There is a need in the art to provide a micromechanical device with piezoelectric actuation that overcomes the drawbacks of the prior art.